The present invention relates to a gradient coil for nuclear magnetic resonance image forming apparatus. It finds its application more particularly in the medical field where the formation of images by nuclear magnetic resonance is unanimously recognized as an aid to diagnosis. It may of course be used in other fields. The purpose of the present invention is to contribute to the creation of images of a body to be examined which are more faithful and more precise in their resolution.
1. Field of the Invention
A nuclear magnetic resonance image forming apparatus has essentially three types of coils. The purpose of a first type of coil (which may possibly be replaced by a permanent magnet) is to create an intense homogenous field B.sub.o in a predetermined space. The purpose of a second type of coil, called radiofrequency coil, is to subject a body examined and placed under the influence of the field of the first coils to sequences of radiofrequency excitation, and to measure a radiofrequency signal reemitted in return by particles of the body. The radiofrequency response is a volume response: all the particles of a region of the body subjected to the examination emit their radiofrequency response at the same time. In order to create the image it is necessary to differentiate these responses. For this, the image forming apparatus comprise a third type of coil called gradient coil, for superimposing on the intense field additional field components. The value of these components depends on the coordinates in space of the place of their application. Conventionally, it is proposed to organize this differentiation along three orthogonal axes X, Y, Z. By convention, axis Z is generally colinear with the intense field created by the first type of coil. In other words, each location of the space may be coded by a different field value: the modifications which result therefrom in the reemitted signal are used.
Gradient coils are consequently divided into three families: that which creates a gradient along X, that which creates a gradient along Y, and that which creates a gradient along Z. The purpose of the invention is more particularly to propose an improvement to the construction of the first two families. A field gradient along X is a magnetic field in which the distribution of the amplitude of the component colinear with the intense field (Z), in space, depends solely on the coordinate x.sub.i of its place of application. In practice, it is even simply proportional thereto. This means that all the particles of a body to be examined situated in a plane parallel to Y-Z and with a given abscissa x.sub.i are subject to a total field B.sub.o +G.sub.x.x.sub.i. The gradient G.sub.x is the slope of the variation of the component along Z of the additional field provided by these gradient coils X.
The acquisition of an image requires during application of the excitation sequences, the joint application of field gradient sequences. The field gradient sequences depend on the image forming method used. This method may for example be of the 2 DFT type perfected by Messrs. A. KUMAR and R. R. ERNST or for example of the retroprojection type perfected by M. P. C. LAUTERBUR. Whatever the image forming method chosen, the particularity of the field gradients is that they are pulsed. They are established, they persist for a short time, then they are cut off. This may happen once or several times during the sequence. This particularity means consequently that the operation of the coils of the third type must be studied not only under permanent operating conditions, during the application of the gradients, but also during transitory conditions resulting from establishment and cutting off thereof. Another particularity of the field gradients relates to their homogeneity. By homogeneity is is meant the compliance, with a given tolerance and for a real field gradient, with an ideal theoretical distribution which it is desired to impose. In fact, the inhomogneities result in falsifying the differentiation which it is desired to impose in space and which is the very base of the principle of image formation. From this point of view the problems of homogeneity must be solved for the gradients as well as they are for the intense homogenous field. The solution of the problems created by the transitory conditions is generally solved by forming three types of coils as small as possible. The smaller they are, the smaller their self-inductance and the smaller the power required for producing the field. On the other hand, for the homogeneity, the larger the coils the more the distribution of the fields which they produce may be considered as homogenous. These two tendencies are therefore contradictory. In general, the gradient coils are placed on the outside and as close as possible to the radiofrequency coils. These latter define therewithin the examination volume where the body is placed.
2. Description of the Prior Art
Such gradient coils are known in the prior art, in particular from the European patent application No. 82 107 453.1 filed on Aug. 16, 1982. These coils include conductors formed about a cylinder along a so-called saddle contour. The coils of the invention are of the same type but have features adapted for solving as well as possible the dual problem of homogeneity and of the electric power to be used for creating field gradients, from field coils whose position is fixed outside a given cylinder and corresponding to an access tunnel provided for introducing the body to be examined therein.